This invention relates generally to golf balls, having at least a center, and a multi-layer cover. The inner cover can comprise polyisoprene, the outer cover can be formed of a polymer blend including a polyurethane composition and the core can include a polybutadiene composition.
Conventional golf balls can be divided into two general groups: solid balls or wound balls. The difference in play characteristics resulting from these different types of construction can be quite significant. Balls having a solid construction are popular with golfers because they provide a very durable ball while also providing maximum distance. Solid balls are generally made with a solid core, usually made of a cross linked rubber, enclosed by a cover material. Typically the solid core is made of polybutadiene which is chemically crosslinked with zinc diacrylate and/or similar crosslinking agents. In addition to one-piece solid cores, solid cores may also contain a number of outer layers, such as in a dual core golf ball. The cover is generally an ionomeric material, such as SURLYN(copyright), which is a tradename for a family of ionomer resins produced by E. I. DuPont de Nemours and Co. of Wilmington, Del. Covers are typically a single layer but may also include one or more layers, such as in a double cover having an inner and outer cover layer.
The combination of the solid core and ionomeric cover materials provide a ball that is very durable and abrasion resistant. Further, such a combination tends to impart a high initial velocity to the ball, which results in increased distance. Because these materials are very rigid, however, solid balls can have a hard xe2x80x9cfeelxe2x80x9d when struck with a club. Likewise, due to their construction, these balls tend to have a relatively low spin rate, which can provide greater distance and increases accuracy off the tee.
Wound balls typically have either a spherical solid rubber or liquid center, around which many yards of a tensioned elastomeric thread are wound. The wound core is then covered with a durable cover material, such as SURLYN(copyright) or similar material, or a softer cover material, such as polyurethane. Wound balls are generally softer and provide more spin, which enable a skilled golfer to have control over the ball""s flight and landing position. Particularly, with approach shots onto the green, the high spin rate of soft, wound balls enable the golfer to stop the ball very near its landing position.
To make wound golf balls, manufacturers use winding machines to stretch the elastic threads to various degrees of elongation during the winding process without subjecting the threads to unnecessary incidents of breakage. Generally, as the elongation and the winding tension increases, the compression and initial velocity of the ball increases. Thus, a more resilient wound ball is produced, which is desirable.
For wound golf balls, the thread is typically formed by a calendar and slitting method rather than an extrusion method. The calendared thread typically has a rectangular cross-section, while extruded thread generally has a circular cross-section.
Prior art wound golf balls and cores typically use polyisoprene rubber thread wound onto the cores at elongations of between 500 to 1000%. The amount of thread required for a golf ball core is dependent on the elastic modulus of the thread in the elongated state. Elongated polyisoprene thread generally has an elastic modulus of 10,000 psi to 20,000 psi. Further, the properties, in particular resilience, of the wound ball or core are dependent on how well the thread packs during winding. The dimensions of the thread and winding pattern control the packing density. Present art polyisoprene threads are typically at least {fraction (1/16)} inches wide by 0.02 inches thick, measured prior to winding. Present art polyisoprene thread, however, is commonly produced in thicknesses between 0.014 inches and 0.024.
U.S. Pat. No. 6,149,535 discloses a thread for winding having at least about 10 individual strands that are each at most about 0.01 inches in diameter. Preferably, the thread has more than 25 strands with diameters of less than about 0.002 inches. The smaller thread dimension allows the thread to be wound more densely. Preferably, the elastic modulus of the thread is greater than 20 ksi when wound about a center. Preferably, the maximum elongation of the thread is greater than about 8%.
A variety of golf balls have been designed by manufacturers to provide a wide range of playing characteristics, such as compression, velocity, xe2x80x9cfeel,xe2x80x9d and spin. In addition to ionomers, one of the most common polymers employed is polybutadiene and, more specifically, polybutadiene having a high cis-isomer concentration.
The use of a polybutadiene having a high cis-concentration results in a very resilient golf ball. These highly resilient golf balls have a relatively hard xe2x80x9cfeelxe2x80x9d when struck by a club. Soft xe2x80x9cfeelxe2x80x9d golf balls constructed with a high cis-polybutadiene may also be constructed, however, they tend to have low resilience. In an effort to provide improved golf balls, various other polybutadiene formulations have been prepared, as discussed below.
The most common polymers used by manufacturers to modify the properties of golf ball layers and/or covers have been ionomers, such as SURLYN, commercially available from E. I. DuPont de Nemours and Co., of Wilmington, Del. Recently, however, manufacturers have investigated the used of alternative polymers, such as polyurethane. For example, U.S. Pat. No. 6,132,324 is directed to a method of making a golf ball having a polyurethane cover. This patent is hereby incorporated by reference.
Polyurethanes have been recognized as useful materials for golf ball covers since about 1960. Polyurethane compositions are the product of a reaction between a curing agent and a polyurethane prepolymer, which is itself a product formed by a reaction between a polyol and a diisocyanate. The curing agents used previously are typically diamines or glycols. A catalyst is often employed to promote the reaction between the curing agent and the polyurethane prepolymer.
Since 1960, various companies have investigated the usefulness of polyurethane as a golf ball cover material. U.S. Pat. No. 4,123,061 teaches a golf ball made from a polyurethane prepolymer of polyether and a curing agent, such as a trifunctional polyol, a tetrafunctional polyol, or a diamine. U.S. Pat. No. 5,334,673 discloses the use of two categories of polyurethane available on the market, i.e., thermoset and thermoplastic polyurethanes, for forming golf ball covers and, in particular, thermoset polyurethane covered golf balls made from a composition of polyurethane prepolymer and a slow-reacting amine curing agent, and/or a difunctional glycol. The first commercially successful polyurethane covered golf ball was the Titleist(copyright) Professional(copyright) ball, first released in 1993.
Unlike SURLYN(copyright) or other ionomer-covered golf balls, polyurethane golf ball covers can be formulated to possess the softer xe2x80x9cfeelxe2x80x9d of balata covered golf balls. Conventional golf ball covers made from polyurethane, however, have not fully matched SURLYN(copyright)-covered golf balls with respect to resilience or the rebound that is in part a function of the initial velocity of a golf ball after impact with a golf club.
U.S. Pat. No. 3,989,568 discloses a three-component system employing either one or two polyurethane prepolymers and one or two polyols or fast-reacting diamine curing agents. The reactants chosen for the system must have different rates of reactions within two or more competing reactions.
U.S. Pat. No. 4,123,061 discloses a golf ball made from a polyurethane prepolymer of polyether and a curing agent, such as a trifunctional polyol, a tetrafunctional polyol, or a fast-reacting diamine curing agent.
U.S. Pat. No. 5,334,673 discloses a golf ball cover made from a composition of a polyurethane prepolymer and a slow-reacting polyamine curing agent and/or a difunctional glycol. Resultant golf balls are found to have improved shear resistance and cut resistance compared to covers made from balata or SURLYN(copyright).
U.S. Pat. No. 5,692,974 discloses methods of using cationic ionomers in golf ball cover compositions. Additionally, the patent relates to golf balls having covers and cores incorporating urethane ionomers. Improved resiliency and initial velocity are achieved by the addition of an alkylating agent such as t-butyl-chloride which induces ionic interactions in the polyurethane to produce cationic type ionomers.
International Patent Application WO 98/37929 discloses a composition for golf ball covers that comprises a blend of a diisocyanate/polyol prepolymer and a curing agent comprising a blend of a slow-reacting diamine and a fast-reacting diamine. Improved xe2x80x9cfeelxe2x80x9d, playability, and durability characteristics are exhibited.
It is thus desired to prepare wound golf balls having lower compression, i.e., a softer ball, while having the same or higher resilience than conventional wound balls. It is alternatively desired to obtain the same or lower compression while achieving greater resilience.
U.S. Pat. No. 5,885,172 discloses a stiff thermoplastic inner cover with a thin thermoset material formed from a reactive liquid material. The thermoset material is formed and reacts at a temperature below the melting point of the stiff thermoplastic layer and no flow of the inner cover to the surface of the ball occurs.
U.S. Pat. No. 6,126,559 discloses a small, soft core with a thermoplastic cover at least 0.142 inches thick, preferably at least 0.150 inches thick, and more preferably at least 0.157 inches thick, and hardnesses of at least 60 Shore D. The thick cover can be molded in one or more layers, each of which may have the same hardness.
U.S. Pat. No. 6,117,025 discloses a three layer golf ball where each layer has at least a 3 point difference in Shore D hardness measured on the ball. An intermediate layer that is softer than at least one other layer is required.
One commercially available golf ball has two layers having the same hardness and the inner layer is not pigmented. Both layers are formed from the same ionomer, each having a Shore D hardness of about 70.
U.S. Pat. No. 6,210,283 disclose a urethane outer cover with an ionomer inner cover.
The invention relates to a golf ball including a core, a cover disposed about the core, which includes an inner cover layer formed from a material including rubber, and an outer cover layer having a thickness of less than about 0.05 inches and including at least one castable reactive liquid material or polyurethane. In one embodiment, the core is solid and comprised of one or more layers. In one embodiment, the golf ball further includes a wound layer including at least one tensioned thread material disposed between the core and the cover, each thread having at least one strand.
In one embodiment, the inner cover material has a hardness substantially stiffer than the outer cover material. In another embodiment, the rubber includes a natural or synthetic polyisoprene material, a blend of two different types of polyisoprenes, or a blend of polyisoprene with at least one additional polymer, or mixtures thereof. Preferably, the at least one additional polymer includes styrene-butadiene rubber, a styrene reinforced resin and/or may include at least one density-modifying filler.
In an additional embodiment, the inner cover layer is vulcanized. In another embodiment, the inner cover layer has an outer surface that is treated by one or more of halogenation, chemical surface treatment, UV radiation, electron beam exposure, microwave radiation, coating, plasma, or corona discharge.
In still another embodiment, the outer cover layer includes at least one thermoplastic or thermoset polyurethane. In yet another embodiment, the outer diameter of the inner cover layer is at least about 1.6 inches and the outer cover layer has a thickness of less than about 0.04 inches.
In yet another embodiment, golf ball further includes a tensioned elastomer material disposed between the center and the cover to form a wound layer. Preferably, the polyisoprene layer is disposed between the outer cover and the wound layer.
In another embodiment, core is solid, polyutadiene based and formed of a center and an outer core layer that is stiffer than the center.
In an embodiment, the rubber layer has a thickness of about 0.01 to 0.03 inches. In an additional embodiment, the cover materials have a Shore D hardness of about 20 to 60 and rubber layer has a hardness of 50 to 70. Preferably, in this embodiment, the rubber layer material has a flexual modulus of at least 50,000 psi.